2. Field of the Invention
The invention relates to a transmitting apparatus and a receiving apparatus which are suitable for use in an automobile cellular telephone terminal which can cope with both of a digital transmission mode and an analog transmission mode.
2. Description of the Prior Art
A conventional automobile cellular telephone uses an analog system whereby an analog audio signal is frequency modulated and is transmitted and received by using a radio wave of, e.g., a band of 800 MHz. According to the automobile cellular telephone of the analog system, however, there is a limitation in expansion of a system capacity and it is difficult to cope with an increase in number of subscribers. A development, therefore, of an automobile cellular telephone of the digital system is being progressed.
Standardization of the specifications of the automobile cellular telephone of the digital type is being progressed in each country. For instance, according to the system of North America, the digital audio signal is high efficient encoded by using a VSELP (vector Sum Excited Linear Prediction) as a high efficient coding using, for instance, a coding system of the analysis synthesizing system as a base. The high efficient coded audio signal is .pi./4 shift QPSK modulated and transmitted by using a band of 800 MHz. A TDMA (Time Division Multiplexing Access) is used as an accessing method. A channel interval is set to, e.g., 30 kHz.
Development of an automobile cellular telephone terminal of the dual mode which can cope with both of the digital system and the analog system is being progressed. A number of signals of different frequencies are needed in such an automobile cellular telephone terminal of the dual mode which can cope with the digital and analog systems.
That is, in the digital system, a clock signal to sample the audio signal is needed. A sampling frequency of the audio signal is set to 8 kHz. A modulation rate in the digital mode is set to 24.3 kHz. In the analog system, wide band data can be transmitted. A data rate of the wide band data is set to 20 kHz. A channel frequency of every 30 kHz is set. Further, a carrier of a digital frequency modulator is needed in the automobile cellular telephone of the dual mode which can cope with digital and analog systems.